1. Field of the Invention
This invention relates to the field of hearing aids. More particularly, the invention relates to an improvement in adaptive feedback cancellation.
2. Background
A common problem with hearing aids is oscillation caused by unstable feedback. Many investigators have described the use of adaptive feedback cancellation (AFC) to solve this problem. AFC may be performed either with a probe noise signal or with the normal hearing aid input. Hearing aid users generally find probe noise to be objectionable, so it is preferable to perform AFC with the normal hearing aid input signal. However, any correlation between the hearing aid input and output signals will introduce bias in the AFC adaptive filter coefficients, thus compromising performance. This problem is particularly severe for tonal input signals, such as music, which are highly autocorrelated.
The bias problem can be reduced by applying processing in the forward path of the hearing aid that decorrelates the output signal from the input signal. The decorrelation processing must be carefully designed to avoid introducing unpleasant auditory artifacts. One method of decorrelation is frequency shifting. In “Adaptive Feedback Cancellation with Frequency Compression for Hearing Aids”, Journal of the Acoustical Society of America, 94(6):3248-3254 (1993), Joson et al. first proposed this method and showed it to be highly effective at reducing bias.
The method described by Joson et al. has the following features:                The frequency shifting ratio is on the order of 6%.        Frequencies are shifted downward (“frequency compression”).        Frequency shifting is accomplished using a “sampling method”, in which the input signal is divided into short segments which are temporally stretched via interpolation and then concatenated with overlapping to produce the output signal.        Interpolation of input segments is accomplished using standard sampling rate conversion techniques.        Frequency shifting is applied to the entire signal, rather than to a band-limited portion of the signal.        
This method may cause objectionable artifacts in four ways. First, any frequency shifting method alters the pitch perceived by the hearing aid user. A frequency shift of 6% corresponds to a musical half-step. For speech, this degree of pitch change may not be objectionable; indeed, Joson et al. found it to be “barely noticeable”. However, music is a much more demanding test signal. Altering the pitch of music by a half-step is highly noticeable by listeners with musical experience.
A second artifact results from acoustic mixing of the processed and unprocessed signals. Because no hearing aid provides a perfectly attenuating seal, some unprocessed signal will leak past the hearing aid and acoustically mix with the processed signal inside the ear canal. Since the processed signal is a frequency-shifted version of the unprocessed signal, the resulting mix may have a distinctly unpleasant sound. For music, it would sound like two musicians playing out of tune with each other.
A third artifact results from the use of the “sampling method” of frequency shifting. This method is known to create artifacts at segment boundaries; additional processing, with consequent added complexity, is required to minimize these artifacts. Even with such additional processing, the method performs poorly for complex inputs such as music. Higher-quality methods of frequency shifting have been devised, particularly for music, but these methods are generally too computationally complex to be implemented under the power, size, and real-time constraints of a hearing aid.
A fourth artifact results from the introduction of a time-varying interaural timing difference (ITD). A frequency shifter, by its nature, is equivalent to a time-varying delay. If a hearing aid user is wearing a frequency-shifting hearing aid in one ear only, a time-varying ITD is created, because the signal received by the aided ear will be delayed, in a time-varying fashion, relative to the signal received by the unaided ear. The same phenomenon will occur if the hearing aid user is wearing frequency-shifting hearing aids in both ears, unless the two aids are synchronized to ensure that they impose exactly the same delay at all points in time. Such synchronization would require a means of communication between the two aids, which would significantly increase the complexity of implementation. The perceptual consequence of a time-varying ITD is the illusion of sound sources moving back and forth between the left and right sides of the user. This occurs because ITD is a strong perceptual cue for lateral position of sound sources.